Photomask inspection and verification by lithography image reconstruction using imaging pupil filters

Abstract

A method and tool for generating reconstructed images that model the high NA effects of a lithography tool used to image patterns produced by a mask. Comparison of the reconstructed images with reference images characterize the mask. The method involves providing a mask reticle for inspection. Generating matrix values associated with a high NA corrective filter matrix that characterizes a high NA lithography system used to print from the mask. Illuminating the mask to produce a patterned illumination beam that is filtered with filters associated with the high NA corrective filter matrix elements to obtain a plurality of filtered beams that include raw image data that is processed to obtain a reconstructed image that is further processed and compared with reference images to obtain mask characterization information.

1. A method for inspecting a surface to identify lithographically significant defects, the method comprising:
 providing a mask reticle configured to achieve photolithographic transfer of a mask pattern onto a substrate using a high numeric aperture (NA) lithography system having a high image numeric aperture (NA); 
 processing the optical parameters of the high NA lithography system to obtain a plurality of matrix values associated with a high NA correction matrix that characterizes the optical parameters of the high NA lithography system; 
 illuminating a portion of the mask reticle with a magnification inspection tool to produce a patterned illumination beam associated with a lithography pattern formed on the target; 
 filtering the illumination beam with a one or more filters to obtain one or more filtered beams, the filters comprising values associated with the matrix values of the high NA correction matrix, so that filtered beams conform to a model of optical performance of the high NA lithography system; 
 detecting the one or more filtered beams and producing raw image data associated with each filtered beam; and 
 processing the raw image data associated with the filtered beams to obtain a reconstructed image. 
 
     
     
       2. The method as in  claim 1  wherein processing the raw image data comprises
 summing the plurality of raw images together to obtain a reconstructed image. 
 
     
     
       3. The method as in  claim 1  wherein processing further comprises
 processing the reconstructed image together with a model of photoresist characteristics to obtain a resist-modeled reconstructed image; and 
 comparing the resist modeled reconstructed image with a reference image to conduct inspection including one of pattern verification and defect detection. 
 
     
     
       4. The method as in  claim 1  further comprising:
 comparing the reconstructed image with a reference to achieve at least one of defect detection or pattern verification; and wherein the reference comprises at least one of:
 a data base model of an intended mask pattern after being processed to include optical proximity correction features and being processed with a lithography process model associated with the lithography system to form a simulated lithographically modeled image; 
 a data base model of an intended mask pattern after being processed to include optical proximity correction features and being processed with a lithography process model associated with the lithography system and further processed with a photoresist model to obtain a simulated resist-modeled image; and 
 
 a data base model of an intended mask pattern. 
 
     
     
       5. The method as in  claim 4  wherein comparing the reconstructed image with a reference comprises:
 obtaining the data base model of an intended mask pattern after being processed to include optical proximity correction features and being processed with a lithography process model associated with the lithography system to form a simulated lithographically modeled image; and 
 comparing the simulated lithographically modeled image with the reconstructed image to detect defects in the mask reticle. 
 
     
     
       6. The method as in  claim 4  wherein comparing the reconstructed image with a reference comprises:
 obtaining the data base model of an intended mask pattern after being processed to include optical proximity correction features and being processed with a lithography process model associated with the lithography system and being further processed with a photoresist model to obtain a simulated resist-modeled image; and 
 processing the reconstructed image with a photoresist model to obtain a resist-modeled reconstructed image; and 
 comparing the simulated resist-modeled image with the resist-modeled reconstructed image to detect defects in the mask reticle. 
 
     
     
       7. The method as in  claim 4  wherein comparing the reconstructed image with a reference comprises:
 obtaining the data base model of an intended mask pattern; and 
 processing the reconstructed image with a photoresist model to obtain a resist-modeled reconstructed image; and 
 comparing the data base model of an intended mask pattern with the resist modeled reconstructed image to verify that the correctness of the mask reticle. 
 
     
     
       8. The method of  claim 1  wherein illuminating the mask reticle with the inspection tool includes
 illuminating the mask reticle with illumination parameters that are matched to illumination optical parameters of the lithography system. 
 
     
     
       9. The method of  claim 1  wherein filtering the illumination beam with a plurality of filters further includes
 introducing phase correcting optical elements to produce a plurality of intensity component images that can be combined to model both the optical performance of the high NA lithography system and the polarization effects of the mask reticle. 
 
     
     
       10. A method for inspecting a surface to identify lithographically significant defects, the method comprising:
 providing a mask reticle configured to achieve photolithographic transfer of a mask pattern onto a substrate using a high numeric aperture (NA) lithography system having a high image numeric aperture (NA); 
 illuminating a portion of the mask reticle with a magnification inspection tool to produce a patterned illumination beam associated with a lithography pattern formed on said portion of the mask reticle; 
 filtering the illumination beam with one or more filters to obtain one or more filtered beams, the filters enabling correction of polarization specific phase change effects induced by the magnification inspection tool and accounting for polarization specific phase change effects induced by the high NA lithography system; 
 capturing a plurality of raw images, each associated with the one or more filtered beams; and 
 processing the raw images together to obtain a reconstructed image that models high NA effects produced by the high NA lithography system. 
 
     
     
       11. The method as in  claim 10  wherein the processing further comprises
 processing the reconstructed image together with a model of photoresist characteristics to obtain a resist-modeled reconstructed image; and 
 comparing the resist modeled reconstructed image with a reference image to conduct inspection including one of pattern verification and defect detection. 
 
     
     
       12. The method as in  claim 11  wherein the processing further comprises
 processing the reconstructed image together with a model of etching characteristics to obtain a resist and etch modeled reconstructed image; and 
 comparing the resist and etch modeled reconstructed image with a reference image to conduct inspection including one of pattern verification and defect detection. 
 
     
     
       13. An inspection apparatus for reconstructing a lithographically significant pattern generated by a stepper, the apparatus comprising:
 an illumination system configured to direct illumination through a mask reticle to produce a patterned illumination beam; 
 magnification optics for magnifying and projecting the illumination beam onto an image sensor, the magnification optics including,
 a pupil plane, 
 a filter that models optical effects produced by a lithographic device used to print image patterns from the mask reticle, the filter located in the pupil plane and arranged to receive the beam to form a filtered beam, the filter comprising values associated with matrix values of a high NA correction matrix that characterizes optical parameters of a high NA lithography system so that the filtered beam conforms to a model of optical performance of the high NA lithography system; and 
 
 an image sensor for receiving the filtered beam and producing an output signal associated with the filtered beam. 
 
     
     
       14. An inspection apparatus for reconstructing a lithographically significant pattern generated by a stepper, the apparatus comprising:
 an illumination source directed through an illumination lens system and illumination aperture onto a mask reticle to produce a patterned illumination beam; 
 magnification optics for magnifying and projecting the beam onto an image sensor, the magnification optics including
 a first stage optical system, 
 a second stage optical system, 
 a pupil plane arranged between the first and second stages of the magnification optical system, 
 a third optical element arranged between the first stage optical system and the pupil plane, the third optical element configured to generate one or more optical beams, 
 one or more filters that model optical effects produced by a lithographic device projection optics of a stepper device used to print image patterns from the mask reticle on a substrate and are located in the pupil plane and arranged to receive at least one of the one or more optical beams to form a filtered beam and, wherein each filter corresponds to a different pupil function defined by the high NA correction matrix; and 
 
 one or more sensors, each sensor arranged to receive at least one of the one or more filtered beams to generate one or more output signals, each output signal associated with one of the plurality of filtered beams; and 
 processing circuitry for
 receiving one or more output signals from the one or more image sensors and 
 processing the one or more signals from one or more filtered beams to obtain at least one reconstructed image associated with the mask reticle. 
 
 
     
     
       15. The inspection apparatus as recited in  claim 14  wherein the circuitry for reconstructing an image associated with the mask reticle from the one or more output signals includes circuitry for
 reconstructing an image associated with a mask reticle image as projected through the lithography device. 
 
     
     
       16. The inspection apparatus as recited in  claim 14  wherein
 the third optical element is configured to generate a pair of optic beams; 
 the one or more filters include a pair of filters wherein each filter is arranged to receive one of the pair of optical beams; and 
 the sensor comprises two sensors, each sensor of the two sensors arranged to receive one of the pair of filtered beams to generate an output signal. 
 
     
     
       17. The method of  claim 1 , wherein the filtering the illumination beam with a one or more filters to obtain one or more filtered beams comprises:
 filtering the illumination beam with two or more filters to simultaneously obtain two or more filtered beams. 
 
     
     
       18. The method of  claim 10 , wherein the filtering the illumination beam with a one or more filters to obtain one or more filtered beams comprises:
 filtering the illumination beam with two or more filters to simultaneously obtain two or more filtered beams.